1. Field of the Invention
This invention relates generally to the fabrication process for making tapered etching edges and to the application of this fabrication process to manufacture electro-optical devices. More particularly, this invention relates to the fabrication process by utilizing a controlled etching technique for controlling the tapered angle of a layer formation for constructing an optical integrated circuits (OICs).
2. Description of the Prior Art
Application of integrated circuit (IC) technology to the fabrication of electro-optical devices, or generally referred to the optical integrated circuit (OIC) technology often limited by a low coupling efficiency of the optical devices fabricated by the OIC technology.
In the past decade, developments in the integrated circuit (IC) technology have resulted in the fabrication of electronic devices with increasing number of layers, smaller dimensions and more complex electric connections. More recently, IC technology for integrating large number of electronic devices on a silicon chip has increasingly been applied to the manufacture of electro-optical devices. Development of optical technology is greatly benefited because a wide variety of compact and durable optical devices are made available by the use of the optical integrated circuit (OIC) technology.
Optical systems comprise many electro-optical devices to perform combination of functions are often made with thin film technique. Usually, the laser diode light source is integrated with functional components such as switches, modulators, photo diode detectors, and waveguides. By the use modern IC technology, all these components can now be integrated on one common OIC chip. Integration of these optical devices on one chip has the advantages that the optical system is more compact and has stable construction and thus more durable. Configuration stability of these OIC systems can greatly enhance the operability and application of these optical systems. Complicate procedures to re-adjust the relative position of individual optical devices for optical alignment on several optical axes and calibration of optical intensity as a function of system configuration are no longer necessary. OICs can also withstand higher degrees of mechanical vibrations and temperature variations. Because of their compact construction and small size, the OICs can be more conveniently placed without the demand of large volumes thus may be suitable for broader variety of applications.
One of the commonly used OICs is a `quasi-hybrid` type which comprises laser diode made from a semiconductor coupled to a silicon based OIC which has a combination of photo diode and wave guide network formed on the silicon substrate. For this type of optical systems, a SiO.sub.2 buffer layer formed on the silicon substrate is often used as a wave guide because of its small index of refraction. Additionally, a layer of higher index material for guiding the light wave can be conveniently formed by applying the well known technique of sputtering on a buffer layer. Generally, the guiding layer composed of higher index material is also coupled to a light-sensitive detection area in the substrate for transmitting the light wave thereto. In order to serve the function as a coupler for light wave transmission, the buffer layer must be formed with a precise tapered thickness to form a coupler window. This type of coupler is generally referred to as a tapered buffer layer coupler.
However, one limitation in the use of OIC devices is the loss of light intensity when the light is transmitted through the tapered buffer layer couplers. There appear to be a correlation between the coupling efficiency and the tapered angle. However, the correlation was not clearly defined and the intensity and the light path of the radiation transmission in this type of OIC devices are not well understood and controlled. The development of OIC technology is therefore limited due to this limitation.
The buffer layer near the coupler window must have precise tapered angle to direct and control the wave transmission through these couplers. Additionally, a loss of light intensity often occurs in transmitting the light through the tapered layer near the coupler window, the efficiency of the wave guide is a function of the tapered angle. A coupling efficiency of more than 90% can be achieved when the tilted angle of the tapered buffer layer is less then five degrees. In order to achieve high coupling efficiency, there is need in the field of OIC manufacture to develop the technique for making the precisely controlled tapered angle of less than five degrees.
The aforementioned limitation is not easily resolved even though the techniques for making tapering edges of different kinds in the process of IC fabrication are well known in the art. Many prior art techniques are available for making tapered angle in forming various IC layers. As early as 1972, R. C. Turnbull disclosed in the article entitled `Tapering Metallurgy Edges` (IBM Technical Disclosure Bulletin, Vol. 15 No. 5, October 1972) a method for forming a tapered edge in fabricating aluminum interconnection metallurgy consists of subtracting etching of blanket aluminum layer employing a KTFR photoresist, an Eastman Kodak's trade secret material. A `subtractive` etching technique is disclosed by Turnball in forming this metallurgy side wall. However, the usefulness of this technique is heavily dependent on the etching solution and the underlined material involved in the etching process. The subtractive etching technique as disclosed by Turnball provides a generalized concept. For application to OIC manufacture, techniques are still to be developed to increase the light wave coupling efficiency in a well defined and controlled manner.
Shahar et al. disclose in U.S. Pat. No. 4,938,841 a method for producing a sloped surface in a semiconductor material. A `dynamic` mask is applied to an area where a slope is to be developed and then a standard mask is applied over the dynamic mask. The manufactured sample which has to be etched is immersed in the etchant where the standard mask is not appreciably etched while the dynamic mask is progressively etched thus developing a gradual slope along the etched edges. Similarly, in another U.S. Patent, i.e., U.S. Pat. No. 4,484,987 by Keyser, entitled `Etching Method` (issued on Nov. 27, 1984), an etching technique is disclosed to develop layered materials to produce features with beveled edges. An anisotropic etch is employed to form wells with vertical walls wherein a double mask of a photoresist layer on an underlying thin film is used to define the limits of the anisotropic and isotropic etches. All these techniques disclose method about the making of beveled edges or the formation of sloped surfaces. However, these techniques do not provide definite method and controllable processes to improve the coupling efficiency such that the difficulties in applying the OIC technology to the fabrication of the optical systems may be resolved.
Therefore, there is still a need in the art of applying the OIC technology to the fabrication of optical systems. First, a dearly defined correlation between the coupling efficiency and the tapered angle of the buffer layer near the coupler window must be established. Secondly, a technique for making such angle in a controllable manner for achieving high coupling efficiency must be developed. These tasks have to be achieved before the difficulties encountered in the prior art can be resolved.